1. Field of the Invention
The present invention relates to a semiconductor device, and particularly relates to a semiconductor device in which a power supply used for an output buffer is different from a power supply used for a control circuit unit for controlling the output buffer.
2. Description of Related Art
A semiconductor device such as DRAM (Dynamic Random Access Memory) is provided with an output buffer for outputting data outside a chip. Since a relatively large amount of current flows in the output buffer at one time, its operation causes a power supply noise. Thus, in many cases, the output buffer uses a dedicated power supply (VDDQ) that is separated from other power supply (VDD). In this case, a signal for controlling the output buffer is transmitted from a circuit using the power supply VDD to a circuit using the power supply VDDQ.
The output buffer is typically configured of a pull-up circuit for pulling up a data I/O terminal and a pull-down circuit for pulling down the data I/O terminal. In this case, the signal for controlling the output buffer is configured of pull-up data for controlling the pull-up circuit and pull-down data for controlling the pull-down circuit. Typically, the pull-up circuit is formed of P-channel MOS transistors and the pull-down circuit is formed of N-channel MOS transistors. Thus, when high-level data is output from the data I/O terminal, both the pull-up data and the pull-down data are at a low level, and when low-level data is output from the data output terminal, both the pull-up data and the pull-down data are at a high level. That is, the signals are in phase.
However, since the in-phase signal easily causes the power supply noise, there occurs a problem that when the pull-up data and the pull-down data are transmitted in phase in a circuit block using the power supply VDD, jitters of the power supply VDD increase. This problem is serious when an impedance of the power supply VDD is relatively high.
As a solution for the problem, there may be employed a method for transmitting the pull-up data and the pull-down data in reverse phase in the circuit block using the power supply VDD as described in Japanese Patent Application Laid-Open No. 2001-110185. With the method, the jitters of the power supply VDD can be reduced.
However, in the semiconductor device described in Japanese Patent Application Laid-Open No. 2001-110185, the pull-up data and the pull-down data are converted from reverse phase to in-phase in the circuit block using the power supply VDD and the signals converted into in-phase are supplied to the output buffer. Therefore, there is a section in which the pull-up data and the pull-down data are transmitted in phase in the circuit block using the power supply VDD. The power supply noise easily occurs in the section.
This problem is not so serious when the output buffer is configured of a small number of transistors as in Japanese Patent Application Laid-Open No. 2001-110185. However, when the output buffer is configured of a large number of transistors connected in parallel like an output buffer having an impedance adjustment function, since a load in the final stage of the circuit block using the power supply VDD is so heavy, the in-phase pull-up and pull-down data in this loaded section is a major cause for the power supply noise. Further, when the unit buffers each having the impedance adjustment function are connected in parallel thereby to enable an impedance to be selected, the above problem is more serious as a result of further increase of the number of transistors connected in parallel.